Engines operating with a variable number of active or deactivated cylinders, also referred to as variable displacement engines (or VDE), may be used to increase fuel economy while optionally maintaining an overall exhaust mixture air-fuel ratio about stoichiometry. In some examples, half of an engine's cylinders may be disabled during selected conditions, where the selected conditions can be defined by parameters such as a speed/load window, vehicle speed, etc. In still other examples, cylinders may be individually and selectively deactivated.
A VDE control system may disable selected cylinders through the control of a plurality of cylinder valve deactivators that affect the operation of the cylinder's intake and exhaust valves. Various mechanisms may be used to enable cylinder deactivation. One example of a cylinder deactivation mechanism includes hydraulically actuated latches coupled to rocker arm assemblies to implement zero valve lift and cylinder deactivation. For example, switching roller finger followers (SRFF) may use hydraulically actuated latches. In these systems, hydraulic pressure (such as pressurized oil from an oil pump) may be used for latch actuation. One example valve deactivation mechanism is shown by Hughes et al in US 20170183982. Therein, valve operation is adjusted via a rocker arm assembly that includes a latch pin mounted on a rocker arm. The latch pin is actuated via an electromagnet. Movement of the latch pin into or out of the rocker arm assembly affects an activation state of the corresponding valve.
The inventors herein have recognized that cylinder valve deactivation mechanisms need to be periodically diagnosed to ensure that the fuel economy benefits of operating in a VDE mode can be extended. In addition, valve operation in deactivatable cylinders may need to be diagnosed to ensure proper switching between active cylinder and deactivated cylinder modes. Diagnostic methods that enable the cylinder valve deactivation mechanism to be directly assessed while switching the cylinder between active and deactivated states are particularly beneficial. However, one issue with such a diagnostic is that it requires the engine to be transitioning states, such as where a given cylinder is activated or deactivated. Since the transitioning is based on engine speed-load conditions, which continually change over the course of a drive cycle, there may be extended periods of a drive cycle where there is no VDE transition. Consequently, the opportunities for performing the diagnostic may be limited. If a VDE mode is actively enforced during the drive cycle to complete a diagnostic, engine performance may be affected. For example, if a VDE mode is imposed at high load conditions, torque demand may not be met. In addition, it may not be possible to complete the diagnostic at high engine speeds due to insufficient time being available to actuate the cylinder valve deactivation mechanism.
In one example, the issues described above may be addressed by a method for an engine comprising actuating, via a solenoid, an electric latch pin of a cylinder valve deactivation mechanism coupled to a valve of a cylinder at a first time while engine speed is less than a threshold; and indicating degradation of the cylinder valve deactivation mechanism based on inferred latch pin movement during the actuating. In this way, VDE mechanism diagnostics may be completed independent of a desired valve opening state.
As one example, an engine system may include cylinders having valves that are selectively deactivatable via a cylinder deactivation mechanism that includes a latch pin mounted on a rocker arm assembly. On a given drive cycle, such as after an engine has been started from rest and while the engine is at or below an idling speed, a VDE mechanism diagnostic may be completed. At the low speeds, there may be sufficient time available to energize and then re-energize (or de-energize) an electric solenoid actuating the latch pin. The mechanism may be actuated in an intrusive manner by an engine controller. Therein, the controller may apply a voltage pulse to energize the solenoid to move the latch pin out of the rocker arm assembly (thereby deactivating the corresponding valve), and then apply the opposite voltage pulse to energize the solenoid to move the latch pin into the rocker arm assembly (thereby reactivating the corresponding valve). Latch pin movement during the actuating is then inferred based on a measured electric current signature, which may include a number and (relative) position of peaks and valleys in the electric current signature, as well as a slope of the current. For example, the presence of latch pin movement may be inferred from a signature that includes a temporary current decrease (e.g., to a valley) as voltage is applied (e.g., a change in the slope of the current). Based on a cam timing during the actuating (such as based on whether a cam coupled to the cylinder valve is on a base circle position or a maximum lift position), and further based on the presence or absence of latch pin movement, the cylinder valve deactivation mechanism may be diagnosed. For example, when the cam is at the base circle, the latch pin is expected to be able to freely move between active and deactivated positions. Thus, if latch pin movement is not observed while at this position, it may be inferred that the mechanism is degraded. As another example, when the cam is at a valve lift position, the latch pin is expected to not be able to move between the active and deactivated positions. Accordingly, if latch pin movement is observed while at this position, it may be inferred that the mechanism is degraded. Accordingly, appropriate mitigating actions may be undertaken.
In this way, by correlating an electric current signature of a solenoid to the movement of a latch pin, a cylinder valve deactivation mechanism may be reliably diagnosed. The technical effect of performing the diagnostic during low speeds conditions, before a change in a state of a cylinder valve has been commanded, is that the diagnostic can be completed before the cylinder valve mechanism is needed. As such, this reduces the likelihood of engine error states (such as misfires) that may be triggered by a degraded VDE mechanism. By intrusively performing the diagnostic at engine idling conditions, when there is sufficient time to enable and disable the mechanism, the likelihood of completing the diagnostic on a drive cycle is improved. By timely diagnosing a VDE mechanism, the fuel economy benefits of cylinder deactivation can be extended over a large range of engine operating conditions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.